1. Field of the Invention
The present invention relates to lithographic apparatus and methods.
2. Description of Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
To project the pattern on the substrate, it is desirable that the patterning device and the substrate be accurately aligned, i.e. positioned with respect to each other. Further, the patterning device and the substrate may need to be aligned with respect to the radiation beam and/or other devices that are included in the lithographic apparatus.
In conventional lithographic apparatus, both the substrate table and the pattern support are provided with an actuator assembly that is configured to accurately position the substrate and the patterning device with respect to a reference point. The actuator assembly may include a long stroke actuator and a short stroke actuator. The long stroke actuator and the short stroke actuator are operatively coupled to enable to move the substrate or patterning device over a relatively large distance using the long stroke actuator and to accurately position the substrate or patterning device using the short stroke actuator. It is noted that an actuator for moving an object over a large distance, such as the long stroke actuator, is by itself generally unsuited for accurately positioning.